LAREDO, Texas (NewsNation) — Questions remain surrounding the big rig found with 73 migrants inside — 53 of them dead — along the edge of San Antonio on Monday.

Many are wondering how the trailer was able to pass an internal checkpoint. 

In Laredo, about 160 miles south of San Antonio, Border Patrol agents told NewsNation if they were to comb through every vehicle, they’d be backing up traffic all day.

Now, some are questioning the efficacy of internal checkpoints and their ability to stop human smuggling.

A study conducted by the U.S. Government Accountability Office found that from 2013 to 2016, only 2% of all apprehensions were made at internal checkpoints, compared to 41% of drug seizures.

Law enforcement sources have said migrants sometimes get dropped off before checkpoint, run around, and then get picked up on the other side.

Watch Robert Sherman’s full report in the player above.